Process of producing conductive images including addition of salicylic acid to developing solutions to counteract emulsion ageing



United States Patent 3,486,893 PROCESS OF PRODUCING CONDUCTIVE IMAGES INCLUDING ADDITION OF SALICYLIC ACID TO DEVELOPING SOLUTIONS TO COUNTERACT EMULSION AGEING Jack F. Strange, Hightstown, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Oct. 3, 1966, Ser. No. 583,968 Int. Cl. G03c 11/00, /30 U.S. Cl. 96-384 2 Claims ABSTRACT OF THE DISCLOSURE In a process for preparing an electrically conductive surface which comprises (a) forming a latent developable silver halide image in an emulsion layer by image-wise exposure; and (b) forming by silver transfer development an electrically conductive silver surface image by (l) immersion in a first developer solution, (2) Washing the layer and (3) immersion in a high-energy inverse transfer developer solution containing a reducing agent, a nucleating agent, and a silver halide solvent; the improvement comprises incorporating with the first developer 0.01- grams of salicylic acid per liter.

This invention relates to developer compositions. More specifically, it relates to developer compositions useful in a process for preparing elements containing electrically conductive silver surface images from photosensitive elements by inverse transfer development.

Typical silver halide photographic emulsions normally develop to black silver images having negligible electrical conductance. For certain purposes, e.g., circuit boards, it is desirable to make electrically conductive silver surface images. Such a process has been disclosed in the assignees Blake U.S. application, Ser. No. 487,031, filed Sept. 13, 1965. The images so produced are superior to others produced via processes disclosed in the prior art in that the silver surface image has far higher electrical conductivity. When the developer compositions disclosed in assignees Strange, U.S. application, Ser. No. 494,940, filed Oct. 11, 1965, are used .in the process of the aforementioned application Ser. No. 487,031, the useful life of the developer composition is vastly increased and the electrical conductivity of the silver surface image is increased.

Films used in the inverse transfer method of forming electrically conductive surface images have been found to be inoperative unless the films are used within six months of manufacture; that is, aged photographic films seem to lose the ability to form electrically conductive surface images via inverse transfer development. This invention discloses developer compositions which restore this ability.

Conventional low solvent photographic developers reduce exposed areas of a silver halide element to a black silver image. When silver halide solvent and a nucleating agent are added to such developers, a conventional negative image forms but, in addition, silver transfer takes place in the unexposed areas by diffusion of silver complex ions to the surface, Where nucleation occurs and subsequent silver complex ions are reduced to an electrically conductive, positive, silver image. Since an inverse or reverse image forms, this type of development is known as inverse transfer development. If the development is too slow, the electrically conductive, surface, silver image will not form. In general, when a soluble sulfide, thiocyanate salt, or their equivalents, are added to a high energy, high solvent developer (one which develops of total developable density in 1 minute) electrically conductive silver surface images are formed. However, films seem to lose the ability to form electrically conductive surface images on aging. Since many films are stored for a considerable period of time before use, a developer composition that can produce silver electrically conductive surface images via inverse transfer development of aged film is very desirable.

A surface is defined as being electrically conductive when the resistance is not greater than 5 ohms per square; resistances as low as .05 ohm per square are obtained using the novel developer compositions and process of this invention.

A novel process, for forming a conductive positive silver surface image via transfer development in aged films in its broader aspects comprises:

(A) Developing a silver halide latent image in a conventional silver halide element with a conventional developer containing salicylic acid; and, sequentially,

(B) Forming on the surface in the unexposed areas of said element, by inverse transfer development, an electrically conductive silver surface image having a resistance of not more than 5.0 ohms per square and a ratio in resistance of greater than 1 10 times between the non-conductive and conductive regions.

This broad process can be varied by adding to the developer (B) a hardening agent which is stable at a high pH, and rapidly crosslinks gelatin. A slight improvement in electrical conductivity is obtained by this variation.

This invention encompasses the sequential use of several developer compositions as set forth above. The first, in its broader aspects, comprises an aqueous alkaline developer solution containing salicylic acid in an amount from about 0.01 to 30 gm. per liter of solution. The second, in its broader aspects, is an alkaline developer solution comprising:

(a) a high energy, silver halide developing agent,

(b) a silver halide solvent,

(c) a nucleating agent selected from the group consist ing of the soluble sulfides, selenides, tellurides, etc. disclosed in assignees Blake, U.S. application, supra, and insoluble sulfides, selenides, tellurides, etc., disclosed in assignees Strange, U.S. application, supra, and

(d) a hardening agent stable at a high pH which rapidly crosslinks with gelatin.

To form electrically conductive images, silver halide within the gelatin is complexed, dissolved, and diffused to the film surface where suitable nucleating and developing agents reduce it to metallic silver. Electrical conductivity results from concentrating the reduced silver halide within an small restricted region at and near the surface so that the silver forms a continuous layer. The ideal film structure for processing to prepare electrically conductive silver surfaces comprises (A) A swollen interior region where the silver complex can rapidly diffuse to the surface, and

(B) A tightly crosslinked or very hard surface so that the complex can be restrained and reduced.

It is not known why photographic films gradually lose the ability to form electrically conductive surface images via inverse transfer development but it is speculated that this phenomenon is related to the known gradual increase in hardness films experience with aging. With aging, normal emulsions such as a (litho) emulsion harden to a greater depth, so that there is less difference in hardness between the surface and the interior regions. This invention permits us to regain from aged film the ideal film structure of fresh, unaged, non-uniformily hardened film. Salicylic acid (a swelling agent) in the first developer swells the film; the hardener in the second developer hardens the surface. In essence, this reduces all films to a common physical structure and eliminates a variable from the process. The hardening agents that can be used within this invention should be rapid acting, stable, and preferably of high molecular weight, such as those disclosed in US. Patent 3,232,761, so that the diffusion through th gelatin is slow.

A dual bath developer as described herein is a necessary embodiment of the invention. An advantage of this invention is that when salicylic acid is used in the first developer, the intermediate water wash between the first and second developer can be eliminated. Another advantage is that salicylic acid increases the litho developer tray life.

This invention will be illustrated further by but is not to be limited to the following examples wherein the solutions are aqueous unless otherwise stated.

Example 1 The first developer, Solution A, of a dual-bath developer is prepared in two portions as follows:

Portion 1:

Water ml 750 Sodium formaldehyde bisulfite g 145 Hydroquinone (anhydrous) -g 40 Salicylic acid g 10 Water to make up to liter 1 Portion 2:

Water ml 750 Sodium metaborate (octahydrate) g 12 Sodium carbonate (Na CO -H O) g 112 Potassium bromide (reagent grade) g 4 Sodium bicarbonate (reagent grade) g 11.2 Sequestrene NA3T (trisodium salt of ethylenediamine tetra-acetic acid) g 2 Glucono delta lactone g 1.2 Water to make up to liter 1 The developer Solution A is then obtained by mixing Portion 1 with Portion 2, mixing in 10 ml. of Triton X-200 (an aqueous dispersion of sodium alkylaryl polyether sulfonate, 28% by weight), and adjusting to pH 10 with 3 N sodium hydroxide. (Note.Solution A should be used within 12 hours of mixing.)

The second developer, Solution B, is prepared as fllows:

Solution B:

Water (distilled) ml 500 Sodium sulfite (anhydrous) g 120 Hydroquinone (anhydrous) g 4 l-phenyl-3-pyrazolidone (reagent grade) g 0.25 Boric acid (H BO (crystals) g 5.5 -nitrobenzimidazole nitrate (one gram dissolved in suflicient ethanol and water, in equal portions, to make u 100 ml. of solution) ml 40 Sodium hydroxide (3 N solution) ml 180 Potassium thiocyanate (30 grams dissolved in sufiicient water to make up 100 ml. of solution) ml 100 Cadmimum sulfide (anhydrous) g 0.75 Water to make up to liter 1 ph is adjusted to 13.3.

A lithographic-type film aged six months was exposed 20 seconds at 24 inches from a #2 General Electric Photoflood lamp operated at 13 volts, AC. The emulsion was a high contrast, silver chlorobromide emulsing (30 mole percent silver bromide and 70 mole percent silver chloride) containing 50 grams of gelatin per mole of silver halide, coated on a polyester base. The exposed film was immersed in Solution A for three minutes, water washed three minutes, drained five seconds, then immersed in Solution B for three minutes, washed with water two minutes, and dried at room temperature. The exposed developed image (that is, a negative image) was black and nonconductive while the normally undeveloped and unexposed areas were electrically conductive reflective silver. The measurement ohms per square was obtained as one-hundreth of the resistance, in ohms, of a line times longer than its width. The probes of a Weston Model 785 Circuit Tester, placed at each end of a surface conductive line that was 100 squares long, showed a resistance of 10 ohms or 0.1 ohm per square, thereby showing that this image was capable of conducting electricity. The background i.e., exposed regions, had an electrical resistance of greater than 3 10 ohms per square (the upper measurement limit of the meter used). A ratio in resistance of approximately l 10 times between the nonconductive coating and the conductive regions will assure little leakage of electrical current into the background regions.

Another piece of the film was processed in the same manner except that salicylic acid was left out of Developer A; neither the exposed nor the unexposed region of the film was electrically conductive.

Example 2 Example 1 was repeated except that 10 ml. of glutaraldehyde solution (25%) was added to Solution B. The silvery, electrically conductive, positive surface image had a resistance of 0.075 ohm per square.

Example 3 Example 1 was repeated except the intermediate water wash was eliminated. The unexposed regions of the film contained a silvery image having a resistance of 0.2 ohm per square while the exposed regions contained a non conductive, grey, silver image.

Example 4 Example 1 was repeated except that the pH of Developer A was adjusted to 10.3 with sodium hydroxide solution. The unexposed regions of the film contained a silvery image having a resistance of 0.2 ohm per square while the exposed regions contained a nonconductive, grey, silver image.

Example 5 Example 2 was repeated except that the pH of Developer A was adjusted to 10.3 with sodium hydroxide solution. The unexposed regions of the film contained a silvery surface image having a resistance of 0.2 ohm per square while the exposed regions contained a nonconductive, grey, silver image.

Example 6 Example 3 was repeated except that the pH of Developer A was adjusted to 10.3. The unexposed region of the film contained a silvery surface image having a resistance of 0.5 ohm per square while the exposed region contained a nonconductive, grey, silver image.

Example 7 Example 1 was repeated except that 40 ml. of glutaraldehyde solution (25%) was added to Solution B. The unexposed region of the film contained a silvery surface image having a resistance of 0.075 ohm per square while the exposed region contained a nonconductive, grey, silver image.

Example 8 Example 1 was repeated except that the film comprised a high contrast chlorobromide (20 mol percent silver bromide and 80 mole percent silver chloride) orthochromatically sensitized emulsion, which contained grams of gelatin per mole of silver halide, overcoated with a clear hardened gelatin layer, containing silica particles on a base coated with a dyed antihalation backing. The unexposed regions of the film contained a silvery surface image having a resistance of 0.1 ohm per square while the ex posed regions contained a nonconductive, grey, silver image.

Processing time can generally vary over a very wide range with little advantage obtained for longer processing periods. The minimum processing time is about one minute in the second developer with no particular advantage realized for processing times longer than three or four minutes.

After development and washing, the film may be dried by any convenient method. Care should be used in drying as excessive temperatures will damage the film, e.g., the conductive image may peel off.

The film support for the emulsion layers treated in the process using the novel developer compositions may be any suitable transparent plastic, for example, cellulose acetate, cellulose triacetate, cellulose mixed esters, etc., polymerized vinyl compounds, e.g., copolymerized vinyl acetate and vinyl chloride, polystyrene, polymerized acrylates, polyesters made according to Alles, US. Patent 2,779,684, and patents referred to therein, the polyethylene terephthalate/isophthalates of Britsh Patent 766,290 and Canadian Patent 562,672 and those obtainable by condensing terephthalic acid and dimethyl terephthalate 'with propylene glycol, diethylene glycol, tetramethylene glycol or cyclohexane 1,4-dimethanol (hexahydro-p-xylene alcohol). The above polyester films are particularly suitable because of their dimensional stability.

Paper especially when coated with a water resisting and oleophilic layer, for example, with a hardened gelatin or superficially saponified cellulose acetate layer is useful as a base material. Matting agents, such as, titanium dioxide, silicon dioxide, barium sulfate, etc., in varying quantities, may be incorporated into the water resisting layers.

It is also possible to use plates consisting of glass, metals, alloys, or metal-oxide coated metals as the support.

The silver halide emulsion may contain optical and chemical sensitizing agents, fog-stabilizing compounds, emulsion hardeners, plasticizing compounds, wetting agents, toners, and matting agents.

A preferred method of film exposure is in a camera through the transparent film base so that there is lateral image reversal. A film does not have to be transparent, as a film with an opaque backing, etc., can be exposed from the front and processed to yield the necessary silver transfer to the unexposed surface region. Likewise, films with or without halation protection can be used.

The silver halide emulsion layer is preferably overcoated with a transparent receptive layer, normally gelatin, sometimes called the antiabrasion layer. The nucleating agents will usually permeate this layer and allow the conductive silver layer to form near and on the surface of this layer. The receptive layer must be a hardened nonremovable colloidal layer permeable to water. Some films contain silica in this antiabrasion layer. While the presence of silica in this layer may improve the process, the silica is not essential to the process.

In the unexposed areas during development, it is believed that the silver halide migrates to or near the surface of the film as a complex ion. The nucleating agents, having permeated the surface region, are the starting points for reduction of the silver halide complex to metallic silver. I

Any nonfogging developer can be used to develop the negative image, but ascorbic acid or litho developers are preferred because their low SO concentrations reduce inverse transfer development onto developed (exposed) regions.

The novel process of this invention allows the revival of aged photographic film so that it may be used to form electrically conductive silver surface images via inverse transfer. Addition of the gelatin swelling agent salicylic acid to the first conventional developer has the advantage that the intermediate water wash can be eliminated in a two step inverse transfer development process. Another advantage of the novel developer composition utilizing salicylic acid is a longer tray life for the developer.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for preparing electrically conductive surfaces which comprises (a) forming a latent, developable silver halide image in a photographic element having a single developable, water-permeable, macromolecular organic colloid-silver halide emulsion layer by imagewise exposure of said layer by actinic radiation; and

(b) forming by silver transfer development an electrically conductive silver surface image having a resistance of not more than 5.0 ohms per square in the unexposed areas of an outer water-permeable, organic colloid layer of said element by (1) immersion in a first developing solution containing a silver halide reducing agent; (2) washing the emulsion layer with water, and (3) immersion in a second, high-energy inverse transfer developer solution containing a silver halide reducing agent, a nucleating agent, and a silver halide solvent; the improvement which comprises incorporating with first developer solution about 0.01 to 30 grams of salicylic acid per liter of developer solution.

2. A process as defined in claim 1 where said high energy silver halide developer contains a gelatin hardening agent.

References Cited UNITED STATES PATENTS 2,384,592 9/1945 Bean 9666-.2 2,392,361 1/1946 Britton et al 260-474 2,444,803 7/1948 Bean 9666.3 X 2,662,822 12/ 1953 Land.

3,033,765 5/1962 King et al. 9638.4 X 3,232,761 2/1966 Allen et al. 9666 GEORGE F. LESMES, Primary Examiner R. E. MARTIN, Assistant Examiner US. Cl. X.R. 

